The Vaginal Ultrasound Cerclage Trial is a multicenter, randomized clinical trial, designed to determine the efficacy of cerclage (a purse- string suture placed around the uterine cervix) for the prevention of spontaneous preterm birth prior to 35 weeks' gestation. We propose to study women at significant risk for recurrent spontaneous preterm birth (those with a prior spontaneous birth at 17-32 weeks' gestation) and who also have an increased risk based on midtrimester ultrasonographic findings of a cervical length <25mm. A consortium of 7 perinatal centers, recognized nationally for their expertise in the conduct of clinical trials, will identify approximately 1000 women with a history of a spontaneous preterm birth at 17-32 weeks' gestation. These women will undergo biweekly vaginal sonography beginning at 16-18 weeks, and ending by 22 weeks. Based on our pilot data, approximately one- third of these women will either have or develop a cervix <25mm and be eligible for randomization to cerclage or no cerclage (control group). The proposed efficacy and sample size for this trial is based on many reports in the literature and particularly on two prospective observational studies of vaginal ultrasonographic data from the multicenter NICHD Maternal Fetal Medicine Units Network. The following specific aims will be addressed: 1. To determine if placement of a cervical cerclage reduces spontaneous birth prior to 35 weeks' gestation in women with a prior early spontaneous preterm birth and with a cervix <25mm in length, as determined by serial midtrimester vaginal sonographic evaluation. 2. To determine the safety of cervical cerclage placement at 16-22 weeks' gestation in the patient population described in Aim#1. 3. (Ancillary Aim) To measure vaginal pH and perform Gram stains of vaginal flora and to collect and store maternal serum and cervicovaginal fluid for later analysis of selected markers to predict preterm delivery in order to: A. correlate these markers with the success of failure of cerclage B. further define possible mechanisms of early spontaneous preterm birth.